What Is Resonance?

We hear the word used a lot, but what is resonance? First, in order to explain we have to explain the terms we will use.

  • A period is the amount of time it takes to complete one cycle
  • The number of cycles in one second is the frequency of an oscillation.
  • Frequency is measured in Hertz, named after the 19th-century German physicist Heinrich Rudolf Hertz
  • One Hertz is equal to one cycle per second.

What Is Resonance?

A resonance occurs when a structure or material naturally oscillates at a high amplitude at a specific frequency. This frequency is known as a structural resonant frequency. Typically a structure will have many resonant frequencies.

A dictionary definition of resonance gives us –

“the state of a system in which an abnormally large vibration is produced in response to an external stimulus, occurring when the frequency of the stimulus is the same, or nearly the same, as the natural vibration frequency of the system.”

When the damping in a structure is small, the resonant frequencies are approximately equal to the natural frequencies of the structure, which are the frequencies of free vibrations of the molecules of the material itself.

Furthermore, an individual resonance is the condition when a natural frequency of a structure or material and the frequency at which it is being excited are equal or very nearly equal. This results in the structure or material vibrating strongly and is the classical resonance state. This resonance state can often lead to unexpected behaviour of the structure or material.

The lowest natural frequency, often called the fundamental frequency, is related to the material of which the structure is made. The greater the mass or density of the material the lower the fundamental frequency of vibration. The natural frequency is also related to the speed that a waveform can propagate through the structure. This is determined largely by the molecular make up of the material. Gas, for example, has many free molecules with high kinetic energy, so the waveform can move quickly through the material. A solid has far fewer free molecules and is much denser, therefore the waveform moves more slowly.

In order to measure a resonance of a structure or material with a Prosig P8000 data acquisition system and DATS Professional signal processing software it is necessary to attach an accelerometer to the structure. It is then required to excite or stimulate the structure with the frequencies that it is normally exposed to in its working life. For example, an automotive car tyre would need to be subject to the frequencies it would encounter whilst in use. This would normally be accomplished by use of a shaker or a large heavy hammer. The tyre for example would need to be tested in isolation, and not connected to anything else like the vehicle suspension or wheel rim as these other parts have their own resonant frequencies and would make the capture and analysis of the tyre resonant frequency difficult.

The measured response from the accelerometer will be relative to the excitation and will only exhibit frequencies that are present in the excitation. The excitation must be an acceptable representation of the normal working frequencies applied to the structure or material. If the structure has a resonance in this frequency range there will be a large peak in the response spectrum. The frequency of this peak will correspond to one of the resonant frequencies of the structure or material. If no peak is detected then the resonant frequencies lie outside the operating range of the structure or material. In order to find the resonant frequencies of a structure or material it may be necessary to apply a wider range of frequency excitation.

what is resonance - fig 1
Figure 1

Figure 1 shows a frequency spectrum, this spectrum is a response of a structure to its excitation. A large spike can clearly be seen at approximately 250 Hz.

what is resonance - fig 2
Figure 2

Figure 2 shows a frequency spectrum, this spectrum as in Figure 1 shows a frequency response. However, Figure 2 shows, using cursors, the exact frequency of the resonance. In this case the resonant frequency is 245 Hz.

This means that this structure should probably not be used if in its working life it will be exposed to this frequency. Figure 2 also shows that if this structure was to be used, and only exposed to 300Hz to 400 Hz or perhaps 0Hz to 200Hz , this particular resonant frequency would not be excited, and therefore the structure would not vibrate abnormally.

Further Reading

What Is Resonance? (part 2) (https://blog.prosig.com/2012/08/20/what-is-resonance-part-2/)
What Is Resonance? (part 3) (https://blog.prosig.com/2013/07/17/what-is-resonance-part-3/)
5 Videos That Explain Resonance (https://blog.prosig.com/2011/09/20/5-videos-that-explain-resonance/)

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James Wren

Former Sales & Marketing Manager at Prosig
James Wren was Sales & Marketing Manager for Prosig Ltd until 2019. James graduated from Portsmouth University in 2001, with a Masters degree in Electronic Engineering. He is a Chartered Engineer and a registered Eur Ing. He has been involved with motorsport from a very early age with a special interest in data acquisition. James is a founder member of the Dalmeny Racing team.
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Armani Shepherd
Armani Shepherd
11 years ago

Dear James,
Your blog and explanations are really helpful – thanks. I’m a music student whose been asked to analyze and write about the Islamic call to prayer. During my research it became evident that in some countries the muezzin (caller) is being replaced by a pre-recorded version of the prayer (adhan). Hypothetically speaking, based upon the ‘sympatethic resonance’ effect might it be catistrophic if all 3,000 mosques in a a city (i.e. Istanbul) played the same adhan five times daily. If the sound was played collectively over loudspeakers could this produce a ‘walls of Jericho’ effect. Presently there is a ‘desynchronization’ of sound as various muezzin-s start calling at slightly different times, with different maqams (scales) and melodies, however I was just wondering…. Could a single call to prayer broadcasted across a city be problematic or when mosques and buildings are designed they automatically are damped so it won’t matter?

I’m a far better musician than I ever have been at physics or chemistry hence my question here…. your reply woud be most helpful.

Regards
Armani

Adrian Lincoln
11 years ago

Further to James’ comments I would add that structural damage is only likely to happen when there is high amplitude,low frequency excitation present(either impulsive or periodic. The “walls of Jericho” phenomenum is more likely to have been caused by the seismic impulses of the army marching in step around the city rather than due to the sound of the trumpets. The main contribution of the brass instruments was probably to improve the synchronisation of the marching soldiers.

Armani Shepherd
Armani Shepherd
11 years ago

Thank you very much. You have both been most helpful. Thanks.

F.Benjamin Franklin
F.Benjamin Franklin
10 years ago

HI,
I conducted a vibration test in a shaker machine. I applied 1 g acceleration and monitored the acceleration of the test piece. A linear sweep wave ranging from 30 Hz to 3500 Hz at 300 sec. The output acceleration curve shows some narrow peak points ( first one is acceleration peaked to 6g in a frequency range of 1110 Hz to 1130 Hz Hz. Second one is 5.16 g in a frequency range of 1325 Hz to 1500 Hz . Third one is 4.6 g in a frequency range of 1500 Hz 1650 Hz). But no significant increase in noise level is observed during this time. But a smoother increment in acceleration curve I observed was for 17 g in a frequency range of 2975Hz to 3495 Hz. Here there was a significant increase in noise level. I conducted a FEA for the same and found first natural frequency at 2900 Hz. My questions are, 1)What is the importance of noise curve? 2) How to interpret the narrow peaks? 3) Whether the first natural frequency is between 2975 to 3495 or prior to this peak like 1120 Hz? Kindly explain.

Benjamin Franklin,
Chennai, India.

Benjamin Franklin
Benjamin Franklin
10 years ago

Dear James,
Thanks for your reply. I did not mention that I conducted the test twice. I intended to tell that those are the narrow peak curves I got when I swept the frequency from 30 Hz to 3500 Hz. Kindly draw a curve between Frequency and acceleration based on my values. Let the acceleration values for frequencies I did not mention be 1 g.I would like to ask you the significance of noise curve. Whether the narrow peaks makes sense even though there is no sign of noise level. Hope you understand my question. Kindly explain.

Ricer
Ricer
10 years ago

Hi James,I found your blog very helpful. I got a question to ask you. At one time of our testing, a resonant vibration at 2072 Hz and 34.26g in a valve had been found by some vibration measurements. We then measured in a separate test the natural frequency of one moving part by exciting the part by a special hammer which comes with a vibration measurement instrument. Its natural frequency is about 2000 Hz. I would like to know how I can find out the exiting frequency that caused the resonant vibration at 2072 Hz in our valve testing. Appreciate your help.

Ricer
Ricer
10 years ago

Hi James, Thanks for the quick response. Let me simplify it to a more general question- if one has a vibration source in the system, say, 100 Hz. It can cause the resonances of 100, 200, 300 Hz,…. Is this correct? If there is a resonant frequency in the system in line with one of these frequencies, a resonant vibration would occur. Is this correct? As I observed a resonant vibration at 2072 HZ, could I say that what had excited the system had a frequency of 2072/n Hz (n is an integer)? Is it correct? Thanks again for your time. Appreciated your help.

vishwas
vishwas
10 years ago

Sir,i wanted to know why we are going to find the fundamental natural frequency? (first mode of vibration) and which mode shape/ which frequency should be considered as natural frequency of a structure when there are infinite natural frequencies.

vishwas
vishwas
10 years ago

sir we can find the natural frequencies of a structure in any software ( STAAD or Nastran-patran)how about the excitation in these cases? without any external excitation how can we get natural frequencies?

Belinda Jiang
Belinda Jiang
10 years ago

Dear Ricer,
Very glad to find your blog here. And now i am finding some materials about “resonance point vibration durability test”. I think your blogs may not focus on reliability test, as a quality engineer, i want to learn the result of this resonnance point, how about its effect to our product after long time working.
And the most difficult question for me to define is the resonance point vibration test cycle and its duration time. How to calculate?
Looking forward to your reply, as i am not a English-speaking people, hoping you can understand my question.
Thank you!

Belinda Jiang
Belinda Jiang
10 years ago

Dear James,
Thanks for your reply. 🙂

Cherri Youns
10 years ago

I found this info is very useful. Thanks for sharing. Do you mind if I mention a few lines written in this article in our website if it’s acknowledged you as the writer and links back to this site? Thank you!

Lin
Lin
9 years ago

Hi James

I just found out this blog and find it very well explained. May I know if the resonance frequency of the structure is known, can we predict/relate how the structure will react during random vibration.
Thank you in advance.

V Negi
V Negi
8 years ago

Its really informative and quite methodical.

Juan
Juan
8 years ago

Hello James,
Can you please explain natural frequency and resonance, and how they can relate.

Your reply would be most helpful

Matt
Matt
8 years ago

Hi James,
I am having trouble understanding what the fundamental frequency is. Is this the frequency at which a material etc will vibrate at once the material is excited. For an investigation I will be doing, I will be dropping a specific object onto a bech for example and using an accelerometer measure the vibrations in the bench. I will then put this data into a fourier transform and analyse the frequencies. Will the momentum of the object hitting the bench affect the fundamental frequency recorded? Also I am wanting to know what harmonics are.

Thanks for your time.
Regards Matt

Brian
Brian
8 years ago

Hi James,

Thanks for your informative post. I’ve been studying noise and vibration control for almost a year and still seeking explanations and answers on how to reduce it’s noise and vibration. My idea about resonance and vibration is elaborated, thanks to your blog.

Mohammed
Mohammed
6 years ago

Hello James , Would you please be so kind as to explain and comment about the phase shift in physical way ? why there is phase shift change at the resonance ? or what is the phase shift in more clear physical way . I would very much appreciate if your answer would be with example to understand the basic principles of this . For clarity I am reading about collocated systems in which I excite them with triangle waveform and peak resonance will occur as inherent property of piezoelectric ?

Dhruvit Modi
Dhruvit Modi
5 years ago

Hi I am working on car brake system. I found out we have resonance and annoying moan noise at 250Hz. I have few questions.

(1) Could I find out which components have 250Hz by performing free-free modal analysis of each component? or I need to perform modal analysis for each component by fixing the component the way it is mounted in vehicle?
Free-free and fixed both provide different frequency values but it is really hard to perform fix modal analysis of each component the way it is fixed in vehicle. Basically my question is how free-free and fixed component modal analysis are useful?

(2) We have narrow peaks at frequency mentioned above. I believe it means two parts are having frequency in range of 250Hz. If I find out which components are having natural frequency at 250Hz then how much frequency shift should I maintain in order to remove resonance. For example if one part has 250Hz then how much natural frequency shift should be maintained for other part to avoid resonance? Is there any common value like 10% or 15% natural frequency shift between 2 parts is require to avoid resonance?

(3) How many surrounding components should be checked to find avoid which part is contributing for resonance. I mean should I check modal for each component of car or should I make it specific to brake assembly and other assemblies which are getting connected to brake assemblies like suspension assembly. How to draw a line for finding out which component could contribute in resonance?

Please let me know your thoughts..

E.O. Richard
4 years ago

Mr James, thanks for your prompt and quality answers to people questions.
I wish to have your direct contact for clarification of issues regarding “RESONANCE” thanks in anticipation. E.O. Richard.

Donald Hartwick
Donald Hartwick
4 years ago

James

What do you call a first resonance point

Donald Hartwick
Donald Hartwick
4 years ago

sorry was not clear what do you consider a resonance two times input ?

Quim Cahayagan
Quim Cahayagan
4 years ago

Hi James,

I am working on steel mill plant and we encounter some problems about vibration. I used the modal analysis to calculate the frequencies and mass participation of different mode shapes. For example the machine has a frequency of 29.7 Hz, based on the results 1) how to determine the natural frequency of the structure to avoid the resonance effect? 2) Do i need to consider all natural frequencies up to 90% mass participation and check it individualy? 3) Using modal analysis what are the criteria to consider to avoid resonance effect on the structure?

Any inputs would help a lot. Thank you!

trackback

[…] How exactly the wall of Jericho collapsed is a mystery. From a scientific standpoint, the wall could have collapsed due to aeroelastic flutter or matching resonance frequencies. For more information on what resonance is, see here. […]

Swapnil Narke
3 years ago

Is every resonating frequency is a natural frequency of a system?

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